Java Collections: The Family

The Big Picture

The Collections Framework organizes Java's data structures.

• Under Collection: List, Set, Queue/Deque
• Map is a related interface, not a Collection
• Common interface lets you swap implementations easily
• Part of java.util since Java 1.2

Every Java program beyond basics uses collections.

How It Works

Collection types are Iterable (for-each); Map is not - iterate its keySet/values/entrySet views.

• List: ordered, allows duplicates, index access
• Set: unique elements, no guaranteed order
• Map: key-value pairs, unique keys
• Queue/Deque: order depends on impl - FIFO queues, deque both ends, PriorityQueue by priority
• Backed by arrays, linked lists, hash tables, or trees

In the Java Landscape

Java 9+ factory methods: List.of(), Set.of(), Map.of()

• These create unmodifiable collections
• Streams (Java 8+) transform collections functionally
• Collections utility class: sort, shuffle, sync wrappers
• Guava and Apache Commons extend the framework
• Concurrent collections in java.util.concurrent

Pitfalls and Best Practices

• Program to interface: List<T> not ArrayList<T>
• Choose implementation by access pattern and data needs
• Prefer List.of() for small immutable collections
• Don't mix mutable and unmodifiable references
• Size collections when capacity is known